Compliments of Firestik® Antenna Company, Phoenix, Arizona USA
Send e-mail (tel) 602-273-7152 (fax) 602-273-1836
The normal recommended procedure for tuning a CB antenna instructs you to check SWR on Channels 1 and 40 and use that information to determine if the antenna is electrically long or electrically short. (See our article "Setting the SWR of your Antenna"). You are further instructed to make the SWR on Channel 1 and Channel 40 the same. When you do that, the SWR will be at its lowest point (the dip) at or about Channel 19. The reason for this method would be to allow you to have the best SWR scenario across all 40 CB channels. But, what if you do not use all CB channels? For instance, suppose the majority of your conversations take place on channel 5, or 10, or 30, etc.. You can tune your antenna with the dip in a place other than channel 19.
For all intents and purposes, a transmitting antenna will be resonant at one specific frequency (even though it is possible for a single antenna to have several resonant frequencies but rarely within the same band). This is the frequency where you will generally find the lowest SWR and realize the highest effective radiated power. If you want to zero in on a specific channel, that is, have the maximum effective radiated power on a specific channel, simply tune your antenna so that your SWR is lowest on that channel.
The process is similar to the standard method. Suppose you wanted your SWR to dip on channel 30. In that case, check SWR on channel 20 (10 channels down) and on Channel 40 (10 channels up) and adjust the SWR until it is the same on both channels. This will, in effect, put the SWR dip at or about channel 30. Likewise, if you wanted the dip to occur on Channel 12, you might use Channel 1 (down 12 channels) and channel 24 (up 12 channels) as your reference points. For other channels, just move up and down the same amount of channels from your desired dip point and use those channels as your reference points.
In any case, with the dip positioned at channel 19, making the antenna longer will move the dip towards the lower channels and making it shorter will shift the dip towards the higher channels.
Compliments of Firestik® Antenna Company, Phoenix, Arizona USA
Send e-mail (tel) 602-273-7152 (fax) 602-273-1836